We analyze optical (UV rest-frame) spectra of X-ray
selected narrow-line QSOs at redshift
found in the
Chandra Deep Field South and of narrow-line radio
galaxies at redshift
to investigate the gas
metallicity of the narrow-line regions and their
evolution in this redshift range.
Such spectra are also compared with UV spectra of
local Seyfert 2 galaxies.
The observational data are inconsistent with the
predictions of shock models, suggesting that
the narrow-line regions are mainly photoionized.
The photoionization models with dust grains predict
line flux ratios which are also in disagreement with most
of the observed values, suggesting that the high-ionization
part of the narrow-line regions (which is sampled by the
available spectra) is dust-free.
The photoionization dust-free models provide two
possible scenarios which are consistent with the observed
data: low-density gas clouds
( cm-3) with a sub-solar
metallicity
(), or
high-density gas clouds
( cm-3) with a wide range of
gas metallicity
().
Regardless of the specific interpretation, the observational
data do not show any evidence for a significant evolution
of the gas metallicity in the narrow-line regions
within the redshift range . Instead, we
find a trend for more luminous active galactic
nuclei to have more
metal-rich gas clouds (luminosity-metallicity relation),
which is in agreement with the same finding in the
studies of the broad-line regions.
The lack of evolution for the gas metallicity of the narrow-line
regions implies that the major epoch of star formation in
the host galaxies of these active galactic nuclei is at .

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